A transmission power amplifier (Power Amplifier: PA) used in a radio communication device especially consumes high power in the communication device. Accordingly, improving power efficiency of the power amplifier is the most important issue in developing communication devices. According to a recent communication standard, performing amplitude modulation has become the mainstream in order to achieve spectral efficiency. This amplitude modulation strictly requires avoidance of signal distortion. Therefore, the power amplifier used in the communication device is operated with deep back-off (i.e. low input power) state in order to improve linearity. However, performance of the deep back-of operation leads to reduction in power efficiency in the power amplification.
In order to achieve both of the improvement of the power efficiency and linearity in the power amplifier, a polar modulation technique has frequently been proposed in recent years. The polar modulation technique uses a polar modulator, an RF (Radio Frequency) amplifier, and a power supply modulator. The polar modulator extracts amplitude modulated components and phase modulated components of a modulated signal (e.g., transmission signal data). Now, the polar modulation technique includes an ET (Envelope Tracking) system and an EER (Envelope Elimination and Restoration) system. In the ET system, the polar modulator outputs an RF (Radio Frequency) modulated signal in which amplitude modulated components and phase modulated components are up-converted to a carrier frequency. In the EER system, the polar modulator outputs an RF modulated signal in which phase modulated components of the extracted modulated components are up-converted to a carrier frequency. Further, the polar modulator outputs amplitude modulated components of the extracted modulated components to the power supply modulator. The power supply modulator modulates power supplied to the RF amplifier according to the amplitude modulated components input from the polar modulator. The RF amplifier amplifies the RF modulated signal input from the polar modulator, and modulates and outputs the RF modulated signal amplified based on the power supply modulated by the power supply modulator.
According to the polar modulation technique, the power supply supplied to the RF amplifier is modulated according to the amplitude of the RF modulated signal input to the RF amplifier. Accordingly, the polar modulation technique reduces power consumption when the voltage output by the RF amplifier is low level.
However, when performing power amplification of the modulated signal using the polar modulation technique, high performance is required in the power supply modulator in order to improve the accuracy of the signal that is output at the last stage. For example, the power supply modulator is required to satisfy characteristics that it accurately performs wide band (i.e. high-speed) operations, output high voltage and signals with low noise because of its wide operation range (i.e. wide dynamic range), and realizes high power efficiency at the same time. Several techniques for satisfying these characteristics are disclosed in Patent literatures 1 to 5.
First, FIG. 20 shows a block diagram of a power amplifier 100 disclosed in Patent literature 1. As shown in FIG. 20, the power amplifier 100 includes an error correction unit 113 that corrects a signal error of a pulse modulation unit 112. The pulse modulation unit 112 is implemented by a switching amplifier with high power efficiency. Then, the pulse modulation unit 112 supplies power to an RF amplifier 111. At this time, the power amplifier 100 corrects, in the error correction unit 113, switching noise generated in the pulse modulation unit 112. In this way, the power amplifier 100 suppresses the influence of the switching noise given to the RF amplifier 111. In short, the power amplifier 100 achieves improvement of power efficiency and wide dynamic range (i.e. low noise) characteristics by implementing the pulse modulation unit 112 and the error correction unit 113 with high power efficiency.
Now, the pulse modulation unit 112, the error correction unit 113, and a low pass filter 114 of the power amplifier 100 will be described further in detail. FIG. 21 shows a block diagram of the pulse modulation unit 112, the error correction unit 113, and the low pass filter 114 of the power amplifier 100. In the block diagram shown in FIG. 21, the pulse modulation unit 112 includes a pulse modulator 150, a switching amplifier 124, an attenuator 125, and an integrator 126, and the error correction unit 113 includes an error amplifier 131, an attenuator 133, and an adder 132.
Each of the part in which the pulse modulation unit 112 and the low pass filter (LPF) 114 are combined, and the error amplifier 131 can be regarded as a voltage source that outputs a desired voltage by voltage feedback. Further, the adder 132 is implemented by a capacitor, and has high pass filter (HPF) characteristics. Therefore, it can be regarded that the part in which the pulse modulation unit 112 and the LPF 114 are combined operates as a voltage source for low-frequency components, the error amplifier 131 operates as a voltage source for high-frequency components, and the adder 132 operates as a high pass filter. In short, the circuit shown in FIG. 21 supplies a voltage obtained by combining the voltage generated by the voltage source for low-frequency components and the voltage generated by the voltage source for high-frequency to the RF amplifier as the modulated power supply.
The power amplifier 100 connects, the voltage source with low output impedance, which is the part in which the pulse modulation unit 112 and the low pass filter 114 are combined, and the error amplifier 131 in parallel, which causes short-circuit between these voltage sources and flow of high unnecessary current. In the power amplifier 100, in order to prevent such a problem, an adder (high pass filter) 132 is inserted between voltage sources to prevent a current from flowing between voltage sources in a desired signal band (i.e. low frequency) with high power density. Further, in the power amplifier 100, the adder (high pass filter) 132 allows an out-of-band current (i.e. high frequency current) to flow in order to suppress out-of-band noise (i.e. high frequency noise). In this way, it is possible to suppress unnecessary current between voltage sources to some extent in the desired signal band while suppressing out-of-band nose. However, according to this system, the signal distortion in the desired signal band (i.e. low frequency) cannot be corrected. Furthermore, when the cutoff frequency of the high pass filter 132 is set to be within the desired band so as to correct the signal distortion within the desired signal band (i.e. low frequency), the unnecessary current between the voltage sources increases, and the power efficiency is consequently decreased. In summary, there is a trade-off between the signal accuracy and the power efficiency in this system.
A method of preventing the problem in Patent literature 1 is suggested in Patent literature 2. FIG. 22 shows a block diagram of a power amplifier 200 disclosed in Patent literature 2. As shown in FIG. 22, in the power amplifier 200, a linear amplifier 202 applies a desired voltage to a load 211. The linear amplifier 202 is of voltage-follower type, thereby it is operated as a voltage source with low output impedance. Further, a desired current is supplied from a switching amplifier 242 to the load 211. The switching amplifier 242 performs pulse modulation control based on a detected current at a sense resistor 208 by a pulse modulator 236, thereby operating as a current source that outputs a desired current. Accordingly, in the power amplifier 200, it can be considered that the linear amplifier 202 is used as a voltage source and the switching amplifier 242 is used as a current source. Then, the power amplifier 200 supplies power to a load (i.e. RF amplifier) by the voltage source and the current source connected in parallel. Further, the power amplifier 200 detects a current Ilin from the voltage source, to control an output current Isw from the current source. Further, the power amplifier 200 uses the linear amplifier 202 having small error with respect to the load 211 as the voltage source, thereby capable of suppressing an error of the output voltage Vout. Further, most part of the power is supplied from the switching amplifier 242 with high efficiency, thereby achieving high power efficiency. Further, since the output impedance of the current source is high, the unnecessary power due to the short-circuit between the voltage source and the current source does not flow.
The similar method as that disclosed in Patent literature 2 is also disclosed in Patent literatures 3 and 4. FIG. 23 shows a power amplifier 300 disclosed in Patent literature 3. In the power amplifier 300, a circuit that includes a buffer amplifier circuit 310 and a class-AB amplifier 322 is used as a voltage source, and a circuit that includes a current sensor 338, a pulse width modulator 340, and a DC/DC converter 324 is used as a current source. Further, FIG. 24 shows a block diagram of a power amplifier 400 disclosed in Patent literature 4. In the power amplifier 400, an analog linear amplifier 405 is used as a voltage source, and a non-linear amplifier 403 is used as a current source. As is similar to the power amplifier 200, in both of the power amplifiers 300 and 400, most part of the power is supplied from the current source with high efficiency, and the error of the output voltage Vout is suppressed by the voltage source with high accuracy, thereby achieving both of the high signal accuracy and the high power efficiency.
Furthermore, Patent literature 5 discloses a power amplifier that uses a linear regulator that amplifies an amplitude modulated signal of low-frequency components, a high-pass filter that amplifies an amplitude modulated signal of high-frequency components, and a high-frequency signal amplifier. The power amplifier disclosed in Patent literature 5 combines signals generated by the linear regulator and the high-frequency signal amplifier, thereby improving the signal accuracy.